This invention relates to miniaturized circuits and more particularly to the transmission line interconnection of these circuits.
A transmission line is generally formed by two conductors. When any two or more circuits are interconnected, by at least one transmission line, a certain amount of signal from one circuit may couple into an otherwise isolated circuit. The resulting induced signal is called crosstalk. For example, when a signal is transmitted to a subsequent circuit across the two conductors from a high frequency signal source, such as an antenna received radio frequency (RF) signal from an RF amplifier to a mixer, in one path, or in a reversed direction, a generated signal from a modulator to a transmitter, in the same radio or transceiver, undesired cross-talk coupling can degrade the performance of the radio transceiver.
If one of the two conductors of the transmission line coupling the circuits is used as a grounding conductor, the circuit is referenced as single-ended and the transmission line is a single-ended transmission line.
Otherwise, two non-grounded conductors form a differential transmission line for a differential circuit. The geometry of the two conductors, and their relative positions, are set to establish a characteristic impedance to properly match the impedance from the first differential circuit to the subsequent differential circuit. A differential signal is applied across the two conductors by one differential generating circuit, and the signal travels down the transmission line to the differential receiving circuit, where the signal is measured as the difference between the two conductors. In other words, a differential circuit generates or receives a pair of complementary signals in a phase-inverted relation with each other, known together as a single differential signal.
The substitution of a differential transmission line for a single-ended transmission line can greatly reduce one type of crosstalk coupling called common-mode impedance coupling. Common-mode impedance coupling in a single-ended (non-differential) transmission line is caused by a non-zero parasitic impedance, generally called a ground-return resistance, that is unintentionally shared by two or more otherwise isolated circuits. This common impedance causes crosstalk. Therefore, by reducing the common or shared impedance of the ground conductor with a non-grounded conductor, common-mode impedance coupling is reduced.
In addition to common impedance coupling between circuits, there could also be capacitive (electric) and inductive (magnetic) coupling between the two conductors of the same transmission line if the two conductors are close together.
In the case of two adjacent transmission lines, differential transmission lines can also reduce the capacitive, or electric field, coupling and inductive, or magnetic field, coupling between the differential transmission lines, relative to single-ended transmission lines, such as cross-talk between the mixer and the modulator, if the distance between the two transmission lines is much larger than the separation between the conductors of one of the transmission lines. In the magnetic field coupling case, the magnetic fields around the adjacent conductor lines of each of the transmission lines will be opposite to each other to substantially cancel each other, thus reducing magnetic field radiation to the surrounding environment.
However, space is generally limited in these miniaturized applications, such as in a radio. Therefore, these differential transmission lines must be close together, which also increase the magnetic coupling and decrease the advantage of differential lines over single-ended lines.
A further reduction in crosstalk, over the plain differential line, for the two close conductors of the same differential line or for two close differential lines, can be achieved by twisting the two conductors to form a twisted-pair differential transmission line. This type of line is commonly implemented with two insulated round wires which are twisted about each other. This type of line can reduce crosstalk by reducing the inductive, or magnetic field, coupling. The crosstalk reduction is achieved by reducing the magnetic loop area of the line, and by changing the orientation of the magnetic field continuously over the length of the line.
An un-twisted pair defines a magnetic, or current, loop area over the entire length of the differential transmission line, with the wires being the long sides of a "rectangle". The area of such a loop defines the amount of current that can be induced in the wires by an external magnetic field, such as from an adjacent conductor.
By twisting the wires together, the loop area is minimized. The dimensions of the wire, and the number of twists per inch define the transmission lines characteristic impedance. Furthermore, the remaining loop is now twisted down the length of the transmission line, so that the normal to the magnetic loop area traces a spiral. When two such twisted pair lines are placed close together, the spiraling normals of the twists reduce the ability for the magnetic fields of one pair to induce a current in the other, such as from the modulator to the amplifier, if the circuits are closely spaced.
When interconnecting miniature circuits, such as printed circuits on a printed circuit board or a flexible circuit substrate, integrated circuits (ICs) on a semiconductor substrate, or hybrid circuits, the concept of differential transmission lines can be applied to reduce crosstalk. These conductors are planar because they lie in one or more planes. The differential transmission line can be implemented with both conductors in the same plane (horizontally configured) or in two different planes (vertically configured). In these situations, a general differential transmission line reduces the common-mode impedance coupling. However, since space is generally limited in these miniaturized applications, such as in a radio, these differential transmission lines must be close together, which increase the magnetic coupling.
The problem of magnetic coupling between two planar differential transmission lines can be solved in a similar fashion as for the twisted-pair wire. In U.S. Pat. No. 5,039,824, Takashima et al use conductive through-holes or vias of a printed circuit board to interconnect and form a twisted-pair equal-width planar structure. However, Takashima did not teach the compensation of losses in his structure. Via connections can cause significant ohmic losses and unbalanced equal-width vertically configured conductors can cause parasitic capacitances in the transmission line. Therefore, it is desired to have a low-loss twisted planar transmission line pair that requires no or minimum interconnections between layers.